!(function(doc, win) {
    var docEle = doc.documentElement,
         evt = "onorientationchange" in window ? "orientationchange" : "resize",
         fn = function() {
             var width = docEle.clientWidth;
            width && (docEle.style.fontSize = 100 * (width / 640) + "px");
         };

     win.addEventListener(evt, fn, false);
     doc.addEventListener("DOMContentLoaded", fn, false);
 }(document, window));